There is a requirement to incorporate capacitive touch sensors into devices such as smart phones, MP3 players, PDAs, tablets, Ultrabook PCs, AIO PCs, etc. Such devices generally have a front transparent cover that is made of glass or plastic onto the rear of which a transparent capacitive sensor is bonded. The capacitive sensor often consists of a substrate made from a transparent material such as plastic or glass on opposite sides of which transparent conductive (TC) materials such as indium tin oxide (ITO) are applied and patterned to form transmit electrode (Tx) and receive electrode (Rx) layers. Alternatively, a single layer sensor can be used which consists of one TC layer applied to the substrate which is suitably patterned and interconnected to form separately addressable Tx and Rx structures.
The cover/touch sensor assembly is attached to the display module which typically consists of a liquid crystal display (LCD). Such an arrangement leads to a cover/sensor/display module that is undesirably thick and heavy. To reduce the thickness and weight it is desirable to form the capacitive touch sensor directly on the cover or integrate the touch sensor into the LCD in some way.
Dual layer sensors that are integrated into LCDs can be of two types: “on-cell” type and “in-cell” type. In the “on-cell” type, the sensor is formed on top of the LCD assembly. In the “in-cell” type, the Tx and Rx layers of the sensor are situated at various places within the LCD structure.
In one case, the Tx and Rx electrodes are formed in TC layers located on opposite sides of the glass substrate which carries the colour filter (CF) assembly and which forms the upper substrate of the LCD. The CF is made of stripes of organic RGB materials deposited within a black matrix (BM) structure and over-coated with an organic planarizing (OP) layer. The TC forming the Tx electrode is deposited on top of the OP layer on the CF and the TC forming the Rx electrode is deposited directly on the rear side of the glass substrate.
In another case, the Tx electrode is buried deeper in the LCD and formed in the TC layer that forms the lower electrode of the LCD in the same plane as the TFTs. In this case, the Rx electrode is formed in a TC layer on one or other of the two sides of the substrate carrying the CF.
For the case where the Tx electrode is situated on the CF substrate and forms the top electrode of the LCD, Tx patterning must be carried out before LCD assembly while patterning of the Rx electrode can take place either before or after LCD assembly. For the case where the Tx electrode is combined with the lower LCD electrode and the Rx electrode is on one or other side of the CF substrate, then this Rx layer can be patterned either before or after LCD assembly.
Hence, for in-cell dual layer sensors it is necessary to form a Tx or Rx electrode pattern in a TC layer situated on top of an organic passivation (OP) layer on top of an RGB CF structure on a glass substrate or to form an Rx electrode in a TC layer on a glass substrate with a CF structure located on the rear side.
In both cases, the usual method to form the electrode structures in the TC layers involves multi-step lithographic processes based on resist exposure and chemical etching of the TC. Such lithographic processes are complex and give rise to defects especially when carried out after the LCD has been assembled. It would be desirable to use laser ablation to form the electrode patterns in the TC layer but if standard laser arrangements are used there is a significant risk that the various layers below the glass substrate on which the TC is provided will be damaged during the laser ablation process.
The present invention thus seeks to provide an improved method which enables laser ablation to be used to form an electrode structure in a TC layer situated on top of a glass substrate without causing significant damage to any of the layers beneath the glass substrate.